1. Field of the Invention
The present invention generally relates to electronic clock circuits, specifically to a method and system for providing clock recovery from a high-frequency data signal, and more particularly to such a method and system that has reduced power dissipation, and acceptable cycle variation (jitter).
2. Description of Related Art
Electronic circuits that provide clock signals are used in a wide assortment of devices, and particularly in computer systems. Microprocessors and other computer components, such as random access memory (RAM), device controllers and adapters, use clock signals to synchronize various high-speed operations. These computer clock circuits often use a phase-lock loop (PLL) circuit to synchronize (de-skew) an internal logic control clock with respect to an external system clock.
A typical prior art PLL circuit 1 is shown in FIG. 1 and includes a phase/frequency detector (PFD) 2, a charge-pump 3, a low-pass filter 4, and a voltage-controlled oscillator (VCO) 5. Phase/frequency detector 2 compares two input signals, a reference signal fref (from the external system clock) and a feedback signal ffb, and generates phase error signals that are a measure of the phase difference between fref and ffb. The phase error signals (xe2x80x9cUPxe2x80x9d and xe2x80x9cDOWNxe2x80x9d) from detector 2 are used to generate control signals by charge-pump 3 which are filtered by low-pass filter 4 and fed into the control input of voltage-controlled oscillator 5. Voltage-controlled oscillator 5 generates a periodic signal with a frequency which is controlled by the filtered phase error signal.
The output of voltage-controlled oscillator 5 is coupled to the input ffb of phase/frequency detector 2 directly or indirectly through other circuit elements such as dividers 6, buffers (not shown) or clock distribution networks (not shown), thereby forming a feedback loop. If the frequency of the feedback signal is not equal to the frequency of the reference signal, the filtered phase error signal causes the frequency of voltage-controlled oscillator 5 to shift (upwards or downwards) toward the frequency of the reference signal, until voltage-controlled oscillator 5 finally locks onto the frequency of the reference; following frequency acquisition, phase acquisition is achieved in a similar manner. The output of voltage-controlled oscillator 5 is then used as the synchronized signal (for internal logic control).
In cases where the incoming data is a self-clocking bit stream, the comparator system may be used to extract (recover) the clock information from the data stream itself. Clock extraction for high-speed serial links is usually accomplished using a VCO with a center frequency N for extracting a clock from a serial data stream modulated at N bits/sec. The VCO may provide multiple phases for oversampling, as discussed in the article by Yang, et al., xe2x80x9cA 0.5 um CMOS 4.0 Gbit/s Serial Link Transceiver with Data Recovery Using Oversampling,xe2x80x9d IEEE JSSC Vol 33, No. 5 (May 1998), or more commonly offer just a single phase for direct detection. An example of a single phase D-type phase detector technique used for non-return-to-zero (NRZ) data is disclosed in the article by Boerstler, xe2x80x9cDynamic Behavior of a Phase-Locked Loop Using D-Type Phase Detector and Nonlinear Voltage-Controlled Oscillatorxe2x80x9d, IBM Technical Report TR 21.1428 (Mar. 21, 1991).
For high-bandwidth applications such as in packet switches, maximizing both frequency and density simultaneously is desired, but this causes power dissipation and/or power density to be a significant problem. CMOS technology limitations can also limit the speed at which clock recovery can be accomplished, and operating the clocks at one-half the baud rate has been reported, as in Ewen et al., xe2x80x9cSingle-Chip 1062 Mbaud CMOS Transceiver for Serial Data Communication,xe2x80x9d ISSCC Digest, Vol 38, pp. 32-33 (February 1995).
In light of the foregoing, it would be desirable to devise an improved method for recovering a high-speed clock from a data signal. It would be further advantageous if the method were to result in reduced power dissipation, while still ensuring acceptable levels of jitter.
It is therefore one object of the present invention to provide an improved clock circuit, such as may be used with a microprocessor or other high-performance computer components.
It is another object of the present invention to provide such a clock circuit which is able to extract a clock signal from a data stream having a high data rate.
It is yet another object of the present invention to provide a method of recovering clock signal from a data stream which results in decreased power dissipation as compared to the prior art.
The foregoing objects are achieved in a method of extracting a clock signal from a data stream, generally comprising the steps of generating a plurality of multiphase clock signals, creating a plurality of error signals, at least one error signal for each of the multiphase clock signals, using the data stream, selecting at least one of the error signals based on a plurality of retime state signals, correcting the multiphase clock signals using the at least one error signal to produce corrected multiphase clock signals, and sampling the data stream using one of the corrected multiphase signals to produce a retimed data signal. The multiphase clock signals may be subharmonics of the data stream. In one embodiment, an UP error signal and a DN error signal are created for each of the multiphase clock signals, wherein the selecting step selects one of the UP error signals and one of the DN error signals, and the selected UP error signal and the selected DN error signal are applied to inputs of a charge pump to correct the clock signals. A multiphase voltage-controlled oscillator may be used to provide the multiphase clock signals. The retime state signals are defined using the synchronization states.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.